Involvement of noradrenergic and serotonergic systems in risk-based decisions between options of equivalent expected value in rats.

Risk perception is an important factor that may mediate risk-based decision-making processes regulated by noradrenergic (NA) and serotonergic (5-HT) systems. Most risk-based decision-making models involve complex factors, such as risk perception or reward value, such that the final decision is the result of the interactions among these factors. However, the contribution of risk perception per se in risk decisions has remained unclear. Therefore, in the present study, we made some modifications to the classical probabilistic discounting task (PDT) to focus on the impact of risk perception and noradrenergic/serotonergic systems on decision-making behavior. Meanwhile, we conducted an elevated plus-maze (EPM) test to detect the correlation between anxiety and choice behavior. In the current study, rats had to choose between a "certain" lever that delivered a certain number of pellets and a "risky" lever that delivered eight pellets in a probabilistic manner (descending: 50%, 25%, 12.5% or ascending 12.5%, 25%, 50% of the time). The long-term rewarding values of the two levers were always identical in each block within each session. According to their baseline performances in choosing the risky lever, rats were divided into the risk-prefer group and risk-averse group. The results showed that there was a significant correlation between open arm time in EPM and risky choice for both descending order and ascending order, indicating that highly anxious rats more often preferred the safe option under risk. Pharmacological stimulation of α2-adrenergic receptors via dexmedetomidine (0.01 mg/kg) decreased the preference of probabilistic rewards in the risk-prefer group, while blocking α2 receptors by atipamezole (0.3 mg/kg) also reduced risky choices. The NA reuptake inhibitor, atomoxetine, increased the preference for risky choices in the risk-prefer group, the effect of which was attained via multiple superimposed doses. Administration of the 5-HT2A receptor agonist, DOI (0.1 mg/kg), increased risk-taking behavior in the risk-prefer group. Taken together, these results suggest that NA may be more inclined to process negative information such as loss or uncertainty in the regulation of risk-related decision making, whereas 5-HT may function primarily to increase risk-taking behavior. Our findings may help to further elucidate how noradrenergic and serotonergic systems differentially affect individuals with different risk preferences in terms of regulating risk perception in risk-related decision making.

Differential effects of intra-RMTg infusions of pilocarpine or 4-DAMP on regulating depression- and anxiety-like behaviors.

Depression and anxiety are associated with dysfunction of the mesolimbic dopamine system. The rostromedial tegmental nucleus (RMTg) is predominantly composed of GABAergic neurons that exhibit dense projections and strongly inhibit mesolimbic dopaminergic neurons, proposed as a major "brake" for the system. Consequently, the RMTg may be a crucial brain region for regulating these emotions. The central cholinergic system, particularly the muscarinic receptors, plays an important regulatory role in depression and anxiety. M3 muscarinic receptors are distributed on GABAergic neurons in the RMTg, but their involvement in the regulation of depression and anxiety remains uncertain. This study aimed to examine the effects of RMTg M3 muscarinic receptors on regulating depression- and anxiety-like behaviors in adult male Wistar rats, as assessed through the forced swim, tail suspension, and elevated plus maze tests. The results showed that intra-RMTg injections of the M1/M3 muscarinic receptors agonist, pilocarpine (3, 10, and 30 µg/side), or the M3 muscarinic receptors antagonist, 4-DAMP (0.5, 1, and 2 µg/side), did not alter the immobility time in the forced swim and tail suspension tests. Additionally, pilocarpine (30 µg/side) decreased time spent in open arms and increased time in closed arms in the elevated plus maze; while 4-DAMP (1 and 2 µg/side) played the opposite role by increasing time spent in open arms and decreasing time in closed arms. These findings suggest that RMTg M3 muscarinic receptors have differential effects on regulating depression- and anxiety-like behaviors. Enhancing or inhibiting these receptors can produce anxiogenic or anxiolytic effects, but have no impact on depression-like behavior. Therefore, RMTg M3 muscarinic receptors are involved in regulating anxiety and may be a potential therapeutic target for anxiolytic drugs.

Relationship between the role of muscarinic M3 receptors in morphine-induced conditioned place preference and the mesolimbic dopamine system.

Opioid use disorder mainly results from functional defects in the brain reward loop, which includs the ventral tegmental area (VTA) and nucleus accumbens (NAc; consisting of shell and core, NAcS and NAcC). Reward effects contribute to opioid use disorder. RMTg M3 receptors play a role in opioid reward by regulating the γ-aminobutyric acid (GABA) neuron activity. Dopamine D1 receptors expressed on GABA neurons regulate opioid reward by mediating the dopamine neuron activity in the VTA. Therefore, we investigated the effect of activating M3 receptors by microinjecting pilocarpine into the RMTg along with activating D1 receptors by microinjecting SKF38393 into the VTA on morphine-induced reward effect, using the conditioned place preference (CPP) paradigm (locomotion was also recorded). We also investigated whether the activation of M3 receptors in the RMTg influenced dopamine release in the NAcS. The results showed that the inhibitory role of RMTg pilocarpine (60 µg/rat) infusions in morphine-induced CPP was reversed by VTA SKF38393 (4 µg/rat) infusions. Moreover, morphine (5 mg/kg, i.p.) increased dopamine release in the NAcS, which was blunted by microinjecting pilocarpine (60 µg/rat) into the RMTg. These results indicate that RMTg M3 receptors mediate morphine-induced reward effect, which is probably related to the dopamine activity within the VTA and NAcS. The relationship between RMTg M3 receptors and the mesolimbic dopamine system could be a potential direction for the treatment of opioid use disorder, but further verification through more comprehensive techniques is needed.

The role of dopamine D1- and D2-like receptors related to muscarinic M1 receptors in impulsive choice in high-impulsive and low-impulsive rats.

The non-selective muscarinic receptor agonist oxotremorine-M has been found to decrease impulsive choice in high-impulsive (HI) rats and increase impulsive choice in low-impulsive (LI) rats, but little is known about the muscarinic M1 receptor agonist N-desmethylclozapine (NDMC). This study investigated effects of NDMC on impulsive choice, and the effect of co-administration of NDMC with the dopamine D1-like receptor antagonist SCH 23390 or D2-like receptor antagonist raclopride on impulsive choice in HI and LI rats, characterized by basal levels of impulsive choice in a delay-discounting task. The results revealed that NDMC (1 and 2 mg/kg) significantly increased impulsive choice in HI, but not LI rats. SCH 23390 significantly promoted impulsive choice in HI rats at 0.01 mg/kg, and in LI rats at 0.0075 and 0.01 mg/kg. Moreover, SCH 23390 (0.005 and 0.0075 mg/kg) significantly inhibited the increase in impulsive choice induced by NDMC (1 mg/kg) in HI rats, whereas the increase in impulsive choice produced by SCH 23390 (0.0075 mg/kg) was significantly reversed by NDMC (1 mg/kg) in LI rats. Raclopride (0.04, 0.08, and 0.12 mg/kg) did not affect choice in both HI and LI rats, but significantly antagonized the increase in impulsive choice induced by NDMC (1 mg/kg) in HI rats. These findings suggest that D1- and D2-like receptors might be involved in different effects of the M1 receptor agonist on impulsive choice between HI and LI rats.

Cholinergic M4 receptors are involved in morphine-induced expression of behavioral sensitization by regulating dopamine function in the nucleus accumbens of rats.

Repeated administration of morphine profoundly influences the dopaminergic and cholinergic systems in the nucleus accumbens [including the shell of the nucleus accumbens (NAcS)]. Further, dopamine release is regulated by the cholinergic system, especially the M4 receptor. Drug priming is one of the main factors that induces relapse in drug addiction. The present study first investigated how activation of the M4 receptor in the NAcS affects the expression of morphine-induced behavioral sensitization, through the administration of an M4 agonist (LY2033298) and antagonist (tropicamide), as well as a combination of an acetylcholinesterase inhibitor and M4 antagonist (huperzine-A + tropicamide). Additionally, the influence of a dopamine receptor agonist, in conjunction with an M4 agonist (i.e., SKF38393 + LY2033298), was also examined. Behavioral sensitization was established by exposure to 5 mg/kg morphine once every three days for a total of three exposures. The expression of behavioral sensitization was challenged by 5 mg/kg morphine. Results showed that (1) microinjection of the M4 receptor agonist LY2033298 (0.2 µg/side), but not the antagonist tropicamide (5, 10, or 20 µM/side) into the NAcS blocked the expression of behavioral sensitization; (2) tropicamide (20 µM/side) reversed the inhibition effect of huperzine-A on this behavior; and (3) SKF38393 (1 µg/side) reversed the inhibitory effect of LY2033298 on the expression of morphine-induced behavioral sensitization. These results suggest that the cholinergic M4 receptor in the NAcS plays an important role in the morphine-induced expression of behavioral sensitization through the regulation of dopamine function in rats.

Effects of noradrenergic and serotonergic systems on risk-based decision-making and center arena activity in open field in rats.

Center arena activity in open field (OF) test is associated with risk-taking behaviors. Noradrenaline and serotonin (5-HT) are neurotransmitters involved in both center arena activity and risk-based decision-making. However, the effects of noradrenergic/serotonergic systems on risk-based decision-making in rats with different center arena activity levels have not been clearly characterized. In this study, we explored the effects of the noradrenergic and serotonergic systems on risk-based decision-making in long center-time (LCT group) and short center-time (SCT group) rats. The two groups were formed based on performance in OF test. Then we tested their risk-based decision-making using probability discounting task (PDT); rats had to choose between "small/certain" lever that always delivered one pellet and "large/risky" lever that delivered four pellets in a probabilistic manner (100%, 50%, 25%, 12.5%). The results showed the SCT group chose the large/risky lever less often in 12.5% block and were more sensitive to loss than the SCT group. α2-adrenergic receptor agonist dexmedetomidine (0.01 mg/kg) decreased the frequency of risky choice, while the noradrenaline reuptake inhibitor (NRI) reboxetine (10 mg/kg) had the opposite effect only in the SCT group. Serotonin-noradrenaline reuptake inhibitor (SNRI) duloxetine (5 mg/kg) decreased preference for the large/risky option only in the SCT group. In contrast, pharmacological manipulations of the serotonin system did not affect the frequency of risky choices. These results suggest that noradrenergic system may be involved in weighing gains and losses for probabilistic discounting. Our findings also provide a better understanding of the involvement of center arena activity in risk-taking.

Muscarinic receptors in the nucleus accumbens shell play different roles in context-induced or morphine-challenged expression of behavioral sensitization in rats.

Both drug-related cues and drug priming are the main factors that induce relapse of drug addiction. Previous research has reported that blockade of the muscarinic receptors could significantly depress addictive behavior, suggesting that the muscarinic receptors might be involved in drug use and relapse behavior. The nucleus accumbens (NAc), especially the shell of the NAc, where the muscarinic receptors are expressed, is critical for craving and relapse. This study investigated the effects of microinfusion of the muscarinic receptor antagonist scopolamine into the NAc shell on context- and morphine-induced expression of behavioral sensitization. Behavioral sensitization was established by exposure to 5mg/kg morphine once daily for five consecutive days. Expression of behavioral sensitization was induced by saline challenge or 5mg/kg morphine challenge. The results showed that: (a) the muscarinic receptor antagonist scopolamine (10.8µg/rat) microinjected into the NAc shell blocked expression of conditional sensitization; (b) acetylcholinesterase inhibitor huperzine-A (0.5 and 0.1µg/rat), but not scopolamine (10.8µg/rat), microinjected into the NAc shell blocked morphine-induced expression of sensitization; and (c) pre-infusion of scopolamine (10.8µg/rat) reversed the inhibitory effect of huperzine-A (0.5µg/rat) on morphine-induced sensitization. Our findings suggest that muscarinic receptors in the NAc shell play different roles in context-induced and morphine-challenged expression of behavioral sensitization.

Huperzine A inhibits immediate addictive behavior but not behavioral sensitization following repeated morphine administration in rats.

Acetylcholinesterase inhibitors are regarded as promising therapeutic agents to treat addiction. The current study aimed to examine the effects of huperzine A, a cholinesterase inhibitor, on behavioral sensitization induced by repeated morphine administration and relapse induced by contextual conditioning. The present study also assessed whether the state-dependency hypothesis may explain the results. Adult rats were divided into four groups (n=8) and intraperitoneally injected with 0.2, 0.3 or 0.4 mg/kg huperzine A or saline (1 ml/kg, control), for 5 days. The effect of repeated huperzine A administration alone on locomotor activity was assessed. For the experiments that analyzed the development of morphine-induced sensitization, 40 rats were divided into five groups (n=8): Saline+Saline, Saline+Morphine, 0.2, 0.3 and 0.4 mg/kg huperzine A+Morphine. Following a withdrawal period of 7 days, all animals were administered saline or morphine, as appropriate. To test the state-dependency hypothesis, the rats in the Saline+Morphine group were injected with saline and morphine, while the other three groups were administered different doses of huperzine A and morphine. To examine the effect of huperzine A on the expression of morphine-induced sensitization, the rats in huperzine A+Morphine groups were injected with appropriate concentrations of huperzine A, and morphine. The current results indicated that the administration of huperzine A alone did not affect locomotor activity, while higher doses of huperzine A inhibited the addictive behavior induced by morphine at the development phase. Additionally, huperzine A administration during the expression phase of morphine sensitization did not inhibit the relapse induced by administration of saline. Furthermore, 0.4 mg/kg huperzine A inhibited the expression of morphine-induced behavioral sensitization. Therefore, the results of the current study do not support the state-dependency hypothesis.

Muscarinic acetylcholine receptor but not nicotinic acetylcholine receptor plays a role in morphine-induced behavioral sensitization in rats.

Background and Aim The cholinergic system can affect drug reward. The present study aimed to examine the roles of muscarinic acetylcholine receptor (mAChR) and nicotinic acetylcholine receptor (nAChR) in morphine-induced behavioral sensitization. METHODS: To analyze the roles of mAChR and nAChR in behavioral sensitization induced by morphine (5mg/kg), seven experiments were designed. Experiments 1 and 2 examined the effects of 3, 1, and 0.3 mg/kg scopolamine and 0.2, 0.1, and 0.05mg/kg scopolamine, respectively, on the locomotor activity when administered alone. Experiments 3 and 4 explored the effect of scopolamine on morphine-induced behavioral sensitization. Experiment 5 studied the effect of mecamylamine on morphine-induced behavioral sensitization. Experiments 6 and 7 investigated the effects of scopolamine+huperzine A and mecamylamine+huperzine A, respectively, on morphine-induced behavioral sensitization. RESULTS: The results revealed that 3mg/kg scopolamine, which significantly enhanced locomotor activity when administered alone, inhibited the acquisition of morphine-induced sensitization. However, mecamylamine (0.5, 1, 2mg/kg) did not have these effects. The co-administration of scopolamine (0.05 mg/kg)+huperzine A (0.4mg/kg) or mecamylamine (1mg/kg)+huperzine A (0.4mg/kg) did not affect the acquisition of morphine-induced behavioral sensitization. Scopolamine (0.05mg/kg) which did not affect the locomotor activity when administered alone, but not mecamylamine (1mg/kg), reversed the acute attenuation effect of huperzine A (0.4mg/kg) on morphine-induced locomotor activity at the acquisition stage and reversed the inhibition of huperzine A on the expression of morphine-induced sensitization. CONCLUSION: The mAChR might play a more important role in morphine-induced locomotor activity and the expression of morphine-induced behavioral sensitization. The mechanisms of mAChR and nAChR were relatively separate in morphine-induced sensitization.

Effects of physostigmine on the conditioned hyperactivity and locomotor sensitization to morphine in rats.

Repetitive exposure to opioids elicits sensitization to its psychomotor stimulating effect and environmental stimuli contribute to this effect. This study first developed a procedure that simultaneously measures conditioned hyperactivity and locomotor sensitization, and then investigated the effects of physostigmine on the development and expression of conditioned hyperactivity and locomotor sensitization in rats. Five groups of rats (10-12 rats each) were conditioned with a conditioned stimulus (CS) for 20 min and then drug or saline paired with CS for 2 h daily for 10 days. Rats were tested 20 min on day 18. On day 25, rats were tested 20 min and subsequently 2 h (immediately after morphine injection). Although the 20 min locomotion was not different among the rats on day 1, rats that received 5 mg/kg morphine during conditioning showed higher locomotion than those received saline or 5 mg/kg morphine in the home cage on day 18 and day 25. Rats received 0.1 mg/kg physostigmine and 5 mg/kg morphine during conditioning showed higher locomotion than those received 5 mg/kg morphine on day 18. On day 25, 0.1 mg/kg physostigmine attenuated the conditioned hyperactivity and expression of morphine locomotor sensitization. In contrast, rats received 0.1 mg/kg physostigmine and 5 mg/kg morphine during conditioning showed higher locomotion during 2 h test period than those received 5 mg/kg morphine. In conclusion, this study established a procedure that simultaneously study conditioned hyperactivity and locomotor sensitization. Physostigmine attenuates the expressions but enhances the development of conditioned hyperactivity and sensitization and the possible mechanisms are discussed.